The present invention relates to a positive electrode active material, particularly to a positive electrode active material for a non-aqueous electrolyte battery. The present invention further relates to a high-capacity and low-cost non-aqueous electrolyte secondary battery having a positive electrode containing a specific positive electrode active material.
In recent years, with the widespread use of cordless and portable AV appliances, personal computers and the like, the need has been increasing for compact, light weight, and high energy density batteries as power sources for driving those appliances. In particular, lithium secondary batteries, as having high energy density, are expected to be dominant batteries in the next generation, and the potential market thereof is very large. In most of the lithium secondary batteries currently available on the market, LiCoO2 having a high voltage of 4 V is used as the positive electrode active material, but LiCoO2 is costly because Co is expensive. Under such circumstances, research has been progressing to investigate various positive electrode active materials as substitutes for LiCoO2. Among them, a lithium-containing transition metal oxide has been wholeheartedly researched: LiNiaCobO2 (a+b≈1) is promising, and it seems that LiMn2O4 having a spinel structure has already been commercialized.
In addition, nickel and manganese as substitute materials for expensive cobalt have also been under vigorous research. LiNiO2 having a layered structure, for example, is expected to have a large discharge capacity, but the crystal structure of LiNiO2 changes during charging/discharging, causing a great deal of deterioration thereof. In view of this, it is proposed to add to LiNiO2 an element that can stabilize the crystal structure during charging/discharging and thus prevent the deterioration. As the additional element, specifically, there are exemplified cobalt, manganese, titanium and aluminum. Here, Table 1 lists composite oxides of Ni and Mn used as a positive electrode active material for a lithium secondary battery in prior art examples.
TABLE 1Prior artexamplesComposition of composite oxide disclosedU.S. Pat. No. 5,393,622LiyNi1−xMnxO2, where 0 ≦ x ≦ 0.3, 0 ≦ y ≦ 1.3U.S. Pat. No. 5,370,948LiNi1−xMnxO2, where 0.005 ≦ x ≦ 0.45U.S. Pat. No. 5,264,201LixNi2−x−yMnyO2, where 0.8 ≦ x ≦ 1.0, y ≦ 0.2U.S. Pat. No. 5,629,110LiNi1−xMnxO2, where 0 < x ≦ 0.2, y ≦ 0.2JP-A-8-171910LiNixMn1−xO2, where 0.7 ≦ x ≦ 0.95JP-A-9-129230LiNixMn1−xO2, where M is at least one of Co, Mn, Cr, Fe, V and Al,1 > x ≧ 0.5, preferably x = 0.15JP-A-10-69910Liy−x1Ni1−x2MnxO2, where M is Co, Al, Fe, Mg or Mn, 0 < x2 ≦ 0.5, 0 ≦ x1 < 0.2, x = x1 + x2,0.9 ≦ y ≦ 1.3
All of the composite oxides disclosed in the above U.S. Patents and Japanese Laid-Open Patent Publications are intended to improve the electrochemical characteristics such as the cycle characteristic of LiNiO2 by adding a trace amount of an element to LiNiO2, while retaining the characteristic properties of LiNiO2. Accordingly, in the active material obtained after the addition, the amount of Ni is always larger than that of Mn, and the preferable proportion is considered to be Ni:Mn=0.8:0.2. As an example of a material having a proportion with a highest amount of Mn, Ni:Mn=0.55:0.45 is disclosed. However, in any of these prior art examples, it is difficult to obtain a composite oxide having a single-phase crystal structure since LiNiO2 is separated from LiMnO2. This is because nickel and manganese are oxidized in different areas during coprecipitation, and a homogenous oxide is not likely to be formed.
As described above, as a substitute material for the currently commercialized LiCoO2 having a high voltage of 4 V, LiNiO2 and LiMnO2 as high-capacity and low-cost positive electrode active materials having a layered structure like LiCoO2 have been researched and developed. However, the discharge curve of LiNiO2 is not flat, and the cycle life is short. In addition, the heat resistance is low, and hence the use of LiNiO2 as the substitute material for LiCoO2 would involve a serious problem. In view of this, improvements have been attempted by adding various elements to LiNiO2, but satisfactory results have not been obtained yet. Further, since a voltage of only 3 V can be obtained with LiMnO2, LiMn2O4 which does not have a layered structure but has a spinel structure with low-capacity is beginning to be researched. Namely, required has been a positive electrode active material which has a voltage of 4V, as high as LiCoO2, exhibits a flat discharge curve, and whose capacity is higher and cost is lower than LiCoO2.
As opposed to this, Japanese Patent Application No. 2000-227858 does not propose a technique for improving the inherent characteristics of LiNiO2 or those of LiMnO2 by adding a new element thereto, but proposes a positive electrode active material composed of a nickel manganese composite oxide which represents a new function by dispersing a nickel compound and a manganese compound uniformly at the atomic level to form a solid solution. That is to say, the prior art examples propose plenty of additional elements, but not technically clarify which elements are specifically preferred, whereas the above application proposes the positive electrode active material which can represent a new function by combining nickel and manganese at about the same ratio.
The following shows the prior art examples disclosing the crystal structure and particle morphology of composite oxides.
TABLE 2Prior art examplesMorphology of composite oxide disclosedJP-A-2000-133262LiaNibCOcMndO2 (0.1 ≦ a ≦ 1.2, 0.40 ≦ b < 1.15,0 < c < 0.60, 0 < d < 0.60, 1.00 ≦ b + c + d ≦1.15, 0 < c + d ≦ 0.60)Content “e” of transition metal in Lilayer is 0.006 ≦ e ≦ 0.150LiaNibCOcMndO2Intensity ratio R is 0.510 ≦ R ≦ 0.700, where“intensity ratio R” is the ratio of thetotal peak intensity of (012) and (006)planes to the peak intensity of (101) planein the X-ray diffraction pattern using CuKα radiationJapanese PatentLiNi(1−x)MnxO2 (0 < x < 0.3)No. 3047693A plurality of minute single crystal(JP-A-7-37576)grains are aggregated to form secondaryparticles with spherical, almost sphericalor elliptical shape.Japanese PatentLiNixMn1−xO2 (M is one or more selectedNo. 3232984from Co, Mn, Cr, Fe, V and Al, 1 > x ≧ 0.5)(JP-A-9-129230)Mixture comprising minute crystalparticles with a unidirectional size of 0.1to 2 μm measured by SEM and secondaryparticles with a unidirectional size of 2to 20 μm comprising a plurality of theminute crystal particlesJapanese PatentPorous spherical secondary particlesNo. 3110728comprising a Li composite oxide composed(JP-A-2000-323123)mainly of Li and one or more elementsselected from the group consisting of Co,Ni and MnMean micropore size obtained from amicropore distribution measured by amercury penetration method is 0.1 to 1 μm.Total volume of micropore with a size of0.01 to 1 μm is not less than 0.01 cm3/g.Mean particle size is 4 to 20 μm and tapdensity is not less than 1.8 g/cc.Inflection point of volume decreasingrate by Cooper plot method is not less than500 kg/cm2.
TABLE 3Prior art examplesMorphology of composite oxide disclosedJapanese PatentLiy−x1Ni1−x2MxO2, where M is one of Al, Fe,No. 3130813Co, Mn and Mg, x = x1 + x2, 0.9 ≦ y ≦ 1.3;(JP-A-10-69910)0 < x ≦ 0.2, x1 = 0, x2 = xin the case of M being Al or Fe; 0 < x ≦ 0.5, x1 = 0,x2 = x in the case of M being Co or Mn; 0 < x ≦ 0.2,0 < x1 < 0.2, 0 < x2 < 0.2 in the case of M being MgDiffraction peak ratio (003)/(004) is notless than 1.2 and the ratio (006)/(101) isnot more than 0.13.BET surface area is 0.1 to 2 m2/gPercentage of Ni3+ to whole Ni is not lessthan 99 wt %.Mean particle size D is 5 to 100 μm.10% of particles has a size of not lessthan 0.5 D and 90% has that of not more than2 D in particle size distribution.According to SEM, spherical secondaryparticles have a rough surface, primaryparticles have a length of 0.2 to 3.0 μm,and mean particle length thereof is 0.3 to2.0 μm.Japanese PatentLiNi-based composite oxideNo. 32333520.75 ≦ FWHM(003)/FWHM(104) ≦ 0.9(JP-A-2000-(FWHM is half peak width of powdered X-ray195514)diffraction usingCuK α radiation.)0.25 ≦ I(104)/I(003) ≦ 0.9 (I is integratedintensity)
As described above, those prior art examples describe the particle size, micropore, specific surface area, primary particle, secondary particle and aggregation of primary or secondary particle of composite oxide particles constituting the positive electrode active material, but they do not at all disclose the details of grains and crystal structure within a primary particle, which is the primary object of the present invention. In other words, there has been no detailed study on the grain and crystal structure of the primary particles of the composite oxide constituting the positive electrode active material.
In view of the above, the present invention is intended to provide an active material comprising a lithium-containing composite oxide with high capacity, excellent rate capacity and longer cycle life by adding nickel and manganese elements with a controlled composition to form solid solution, and controlling the crystal structure and superlattice structure as well as the grain arrangement and crystal domain within the primary particles of the oxide at the same time.